The present invention concerns a method for the post-treatment of a metal surface pretreated with phosphating solution, wherein a surface to be painted, in particular, to be cathodically electropainted is phosphate treated prior to painting and the surface thus treated is subjected to cathodic electrolysis treatment by dipping it into chromating solution or with flow coating of chromating solution which provides the surface with excellent corrosion resistance and paintability.
It is a generally known technology to apply chromium-based treatment as a post-treatment after phosphate treatment in order to enhance the corrosion resistance and adhesion of subsequently applied paint film. As a post-treatment solution, chromium-free solutions have been developed in various types as alternatives for chromium-based solutions. In all these post-treatment methods, the surface to be treated is either sprayed with the post-treatment solution or dipped into it.
In the process of phosphate treatment of a car body, chromium-based rinsing has been usually employed in western countries. In Japan, in contrast, chromium rinsing has not been employed for the reason that it requires an additional treatment stage together with waste solution treatment for disposal; this entails increasing expense.
While the adoption of chromium-based rinsing in the pretreatment process prior to electropainting, particularly cathodic electropainting, can afford improved corrosion resistance and paint adhesion after painting, it may still cause paint film blister on surface areas where rinsing solution exists in concentration if the work being treated, in the case of car body specifically, is not water-rinsed after chromic acid treatment. Therefore water rinsing is an indispensable condition. However, water rinsing may reduce the corrosion resistance in half. For this reason there is a limit to the benefits to be obtained from chromium-based rinsing, so that corrosion resistance performance is greatly dependent on the quality of zinc phosphate coating formed in the phosphating process prior to chromium-based rinsing; specifically in the case of cathodic electropainting.
As publications that concern zinc phosphate conversion treatment, there are, for example, Japanese Patent Publications Sho-58-11515/1983 which discloses a treatment solution for forming zinc phosphate coating as a base coating suitable to cathodic electropainting and Japanese Patent Publications Sho-58-11513/1983 and Sho-58-11514/1983 which disclose alternate compositions and methods for the treatment solution. In practical application of these inventions, however, there is a problem that a high quality coating, that is, of dense, plate-like crystals and having alkali resistance as claimed by the inventors of the above patents, is hard to form with stability on a car body surface exterior as well as interior, which may accordingly give rise to variation of corrosion resistance and paint film adhesion after painting.
The corrosion resistance of car is of high importance for the car body underside. The car body exterior such as the fender, door, quarterpanel etc. also requires superior paint adhesion which is of essential importance. In both cases high quality is required and in order to satisfy such requirements it is necessary to effect stringent control on the conditions of the phosphate treatment process.
The present invention aims at solving the aforementioned problems and intends to provide an excellent method of post-treatment for a car body pretreated with phosphate particularly for one to be followed by cathodic electropainting.